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We investigate the chemical evolution of a collapsing core that starts from a hydrostatic core and finally forms a low-mass protostar. New multiphase gas-grain models that include bulk diffusion and photon penetration are simulated by the macroscopic Monte Carlo method in order to derive the chemical evolution. There are two types of species in the ice bulk in the new multiphase models: interstitial species can diffuse and sublime at their own sublimation temperatures, while normal species are locked in the ice bulk. Photodissociation rates of icy species are reduced by the exponential decay of UV flux within the ice mantle. Two-phase models and basic multiphase models without bulk diffusion and photon penetration are also simulated for comparison. Our physical model for the collapsing core is based on a 1D radiation hydrodynamics model. Abundant icy radicals are produced at around 10 K in the new multiphase models. Interstitial radicals can diffuse inside ice mantles to form complex organic molecules (COMs) upon warming up. Thus, COMs produced by radical recombination at higher temperatures in the new multiphase models are more than one order of magnitude higher than those in the two-phase and basic multiphase models. Moreover, COMs produced at around 10 K in the new multiphase models are about one order of magnitude higher than those in the two-phase model. Our model shows a reasonable agreement with observations toward low-mass protostars. Molecular oxygen abundances predicted by our new multiphase models agree reasonably well with those found in cometary materials.

The Chemical Evolution from Prestellar to Protostellar Cores: A New Multiphase Model with Bulk Diffusion and Photon Penetration